Reactive alloys might be determined as alloys which exhibit an increase in chemical interaction with oxygen, nitrogen, carbon, etc. at elevated temperatures. Titanium aluminide, high strength titanium alloys, nickel aluminide, beryllium alloys, refractory metals, zirconium alloys, niobium, and many other pure metals represent the group of such reactive alloys. Thin sheets or foils of reactive metals such as titanium aluminides are used for manufacturing important structural elements designed for aircraft and space applications and the like, where high service temperature and high strength-to-weight components are required. However, this type of titanium alloy is difficult to process into foil or thin sheet elements using hot forming because their oxidation drastically increases at elevated temperatures. Also, the undesired diffusion of a gas into a metal surface produces a decrease in ductility.
The need for elevated temperatures during reactive metal processing has produced a number of prior techniques which eliminate oxidation atmospheres from the environment of the metal during high-temperature processing. For example, hot working in large vacuum chambers or in inert gas environments is a common technique. However, the costly manufacturing facilities, which are required in these processes, add additional costly expenses to the final product. In many applications, an oxide layer is removed from a metal section by machining or the like.
Most technologies known for manufacturing thin sections or foils of reactive metals incorporate special coatings, claddings or capsules that protect the reactive metal workpieces from oxidation and degradation during the hot forming process. For instance, in U.S. Pat. No. 3,164,884 to Noble et al., a method for the multiple hot rolling of sheets is disclosed in which cover plates and sidebars are assembled around inner reactive metal plates separated by a release agent. The sidebars are welded along their outer edges to the cover plates and to each other. The release (separating) agents are water mixtures of aluminum, chromium, or magnesium oxides. Additionally built-in vent holes permit gases that are formed in the package to escape during the hot rolling process.
In U.S. Pat. No. 5,121,535 to Wittenauer et al., a method of forming a reactive metal workpiece was created, which is protected from high-temperature oxidation during hot working by placing the workpiece in a malleable metal enclosure with a film of release agents interposed between major mating surfaces of the reactive metal section and the metal jacket. In a preferred embodiment, a metal section of a reactive metal is placed in a non-reactive metal frame. The reactive metal section and frame are then interposed between non-reactive metals of the top and bottom plates, with a release agent which exhibits viscous glass-like properties at high temperatures being disposed at the interfaces of the reactive metal sections. The release agent is provided preferably in shallow depressions or pockets in the non-reactive sections where the metal interfaces. The assembly, is then welded together near the perimeter so that the release agent is sealed in place between the sections.
The welded assembly may then be hot rolled under pressure to the desired gauge using conventional hot rolling machinery and procedures to form thin metal sections or foils. Other hot working techniques may be employed where suitable. As the assembly is hot rolled, the release agent flows to form a uniform interfacial film. Thus, accelerated oxidation during the high-temperature hot working of the reactive metal section is prevented using the present invention, by encapsulating the reactive metal section in a non-reactive metal jacket during hot working, with the major surfaces of the reactive metal core being separated from the encapsulant layers by a release agent.
Thereafter, the formed assembly or laminate is cooled, and the rolled assembly is sheared to remove the welded edges. The non-reactive metal sections are simply peeled from the reactive metal core by virtue of the presence of the brittle, non-cohesive release agent. Residual release agents can be removed from the finished reactive metal foil by a rinse or the like. In this manner, U.S. Pat. No. 5,121,535 provides a method by which bulk quantities of reactive metals such as refractory metals can be formed into thin metal sections such as foils or strips without the use of vacuum processing equipment and with the utilization of conventional hot working equipment such as hot rolling machinery.
All prior technologies of fabricating thin sheets or foils from reactive alloys have considerable drawbacks which make them undesirable in terms of sufficient protection from oxidation, cost, and production capacity, especially if the thin sections were produced initially from reactive alloy powders, which require additional hot working cycles for compacting. Developed porosity causes very rapid oxidation of the reactive alloy to a substantial depth, and capsules designed in known inventions do not protect the sintered section from rapid oxidation. A significant difference in structures and mechanical properties between sintered sections, produced from reactive powder metal, and the frame (capsule), produced from non-reactive wrought metal, result in nonuniform deformation and stress concentration of the laminate package during the hot rolling process. Cracks occurred in various places of the sintered section during the first cycles of hot rolling and do not allow it to maintain a stable manufacturing process. Therefore, it would be desirable to provide a costeffective method of producing thin metal sections from powder reactive alloys which reduces or eliminates destructive oxidation during high-temperature processing. The present invention achieves this goal by providing a method by which the powder of reactive metals can be formed into fully dense thin sections in a hot working process which can be carried out in an unmodified atmosphere at ambient pressure.